A variety of polymeric materials is suitable for the manufacture of micro and ultraporous membrane filters having symmetrical or asymmetrical pore structures, particularly with the regenerating bath method.
Depending upon the manufacturing conditions and the respective polymer material, membrane filters are hydrophilic (water wettable without the use of a surface-active agent) or hydrophobic (not wettable). Hydrophilic membrane filters of aliphatic polyamide, such as PA 6.6 and PA 6 (e.g., U.S. Pat. Nos. 3,876,738 and 4,340,479 and German Patent Specification 3028213) are of particular interest since they have high chemical and temperature resistance. It is important that the compounds forming the alipathic polyamides have safe toxicological qualities.
The manufacture of these membranes on a commercial scale is generally done in such a way, that a solution of the material forming the membrane, such as a solution of a polyamide in formic acid, that may contain a certain amount of a non-dissolving agent for the polymers is used. The resulting pourable solution is applied, often on a rotating drum, in the form of an endless film onto a backing, and fed into a regenerating bath, where the dissolved polymer is precipitated to form a membrane filter. The filter thus formed is thereafter removed, washed and dried in a continuous strip.
The regenerating bath may contain a certain amount of the same solvent that is used in the manufacture of the pourable solution. The difference in the manufacture of the pourable solution between the solvent material system and the regenerating bath system is that the regenerating bath contains more non-dissolving agents than the solvent system for the manufacture of the pourable solution to effect the precipitation of the membrane filter.
The membrane filters with an approximate thickness between 60 .mu.m and 200 .mu.m, are generally processed by cutting, rolling, pleating, and the like, for possible use in tube filters. The disadvantage of this processing is the lack of mechanical firmness. It is also known that membrane filters can expand when wetted with common solvents, such as water or alcohol.
Because of this characteristic it is known to reinforce membrane filters with mechanically strong, open-meshed, textile reinforcing material, into which the porous membrane filter matrix is imbedded. Spun yarn or staple fiber, or possibly woven or knitted cloth can be used as textile reinforcing material. Perforated foils (ductile foil) can also be used. Textile reinforcement for membranes of PA 6 and PA 6.6 is exclusively made of polyester, such as polyethylterephthalate, and polyolefins, such as polypropylene. There is literature describing the use of fibers of polyamides and aromatic polyamides (aramides), cellulose, such as paper, and regenerated cellulose, cellulose esters and cellulose ethers, such as glass fiber as reinforcing material for membranes of PA 6 and PA 6.6 (U.S. Pat. No. 4,340,479, EP0173500 and EP0096306). Accordingly, the material used for reinforcing membrane filters is generally a substance other than the material of the membrane filter.
It is generally known that the mechanical firmness of membrane filters can be increased and their expansion limited. However, the essential properties of the textile reinforced membrane filters are impaired. Polyolefin material, such as a polypropylene cloth, imbedded into a hydrophilic membrane filter matrix, reduces the frequently desired high processing temperature.
Textile reinforcing material of polyester is not hydrolytic at high pH levels and/or at high temperatures, so that such membrane filters, possibly of polyamides, cannot be used in alkaline media. Polyester reinforcing material is not resistant to a number of solvents, so that a certain amount of dissolved polyester is consequently filtered with the filtering media. This presents considerable problems in the foods and pharmaceutical industry.
Textile reinforcing material of polyolefins, such as polyethelene or polypropylene, has the disadvantage that it is not very temperature resistant, so that the favorable temperature resistance of the filter is reduced if such textile is imbedded into a polyamide membrane filter.
The object of the invention is to produce textile reinforced membrane filters that do not show any of the above described disadvantages of textile reinforced membrane filters of the prior art.